Original articleClinical application of laser evoked potentials using the Nd:YAG laserApplication clinique des potentiels évoqués laser Nd:YAG.
Introduction
The laser stimulation and the cortical responses evoked by it, so-called ‘laser evoked potentials’ (LEPs), were introduced in research in the mid ‘70s in the attempt of better understanding the mechanisms of nociception [14]. The most important achievement from the application of LEPs as a clinical tool is the possibility to investigate objectively some peripheral and central pathways which were prior unexplored in clinical neurophysiology practice 4, 9, 18, 19. Compared to electrical stimulation used for standard somatosensory evoked potential (SEP) testing, which stimulates the large diameter, A-beta nerve fibres, the interest of laser stimulation is to stimulate the small-diameter, thinly myelinated, A-delta nerve fibres and the unmyelinated C-fibres 12, 26, 34, 41, 42, 51. Regarding the central nervous system pathways, the LEPs allow the investigation of the spinothalamic tract 16, 29, 46, whereas the dorsal columns are explored by electrical SEPs. At present, the clinical application of LEPs for the diagnosis of peripheral or central nervous system diseases is just emerging 9, 13, 31, 34.
A CO2 laser (wavelength 10.6 μm), with pulse duration ranging between 10 ms and 60 ms, is classically used to perform LEPs 10, 11, 30. Lasers with longer pulse duration (200 ms) have been proposed as stimulant, i.e. argon laser (wavelength 0.5 μm) 3, 5, 7 and diode laser (wavelength 0.98 μm) [24]. In contrast, a Thulium (Tm):YAG laser (wavelength 2 μm) with short pulse duration of 1–2 ms was recently introduced to produce LEPs 17, 35, 49. Brief CO2 laser pulses (100 ns) [44] and Neodinium (Nd):YAG laser pulses [50] have been used for reaction times and pain threshold measurements, and we were the first to apply a laser beam with an ultra-short pulse duration (5 ns) delivered by a Q-switched Nd:YAG laser (wavelength 1.06 μm) to obtain LEPs. This original approach has been validated in a previous study performed on healthy subjects [38].
A brief laser pulse may be an advantage by leading to shorter activation times, better synchronization of the afferent volley and greater spatial-temporal summation at central synapses. This may contribute to the occurrence of LEPs with shorter latency following a ns-range Nd:YAG laser pulse than following a ms-range CO2 laser pulse. However, a high peak power emission should be available to reach the amount of energy necessary to activate the small nerve fibres and/or their receptors. This condition is fulfilled by the Q-switched arrangement of the Nd:YAG laser. The high power emission obtained with a Q-switched or a mode-locked Nd:YAG laser is able to induce a strong electric field in tissue, that could be the main mechanism to activate the mechano-thermal nociceptors and to produce a pinprick pain sensation, preferentially to the radiant heating which is rather provoked by a CO2 laser stimulation. Other remarks are inherent in the use of a Nd:YAG laser. For instance, the Nd:YAG laser beam penetrates deeper in the skin layers than the CO2 or even the Tm:YAG beam, since skin transmittance increases as laser wavelength shortens within the near-infrared range. Therefore the energy density at the air/skin interface necessary to produce LEPs may be slightly higher for the Nd:YAG laser than for the CO2 laser (10–14 mJ mm–2 vs 8–12 mJ mm–2) 6, 10, 21, 45, but owing to a lower tissue absorption, the energy dissipation per unit of volume into the skin may be weaker for the Nd:YAG laser than for the CO2 laser.
Besides the fact that the exact mechanisms of LEP production by Nd:YAG laser remain to be precised, the aim of the present study was the attempt to verify the clinical usefulness of the Nd:YAG LEPs in the investigation of peripheral neuropathies and spinal cord lesions.
Section snippets
Patients and methods
Six patients with a suspected peripheral sensory neuropathy involving small diameter nerve fibres, but no clinical signs or history of central nervous system disorder (3 women and 3 men, mean age: 64 years, range: 48–79 years) and 6 patients with a suspected unilateral spinothalamic tract lesion (1 woman and 5 men; mean age: 42 years, range: 26–63 years) but no clinical signs or history of a peripheral nervous system disorder were included.
The diagnosis of peripheral neuropathy or spinothalamic
Results
The electrophysiological findings are presented in Table 2 and examples of LEPs in Fig. 1. LEPs following Nd:YAG stimulation of the feet were bilaterally absent in all patients with peripheral neuropathy and at least unilaterally in all patients with spinothalamic tract lesion. LEPs were found bilaterally absent in one patient (n° 9) with spinal lesion. Another one (n° 8) presented absent LEPs for one side, whereas LEPs of the other side had low amplitude (4 μV) and prolonged latency (255 ms)
Discussion
The use of Nd:YAG laser stimulation to produce LEPs appears to be feasible in clinical practice, and extremely sensitive both to the detection of peripheral small-diameter nerve fibre dysfunction and to that of central spinothalamic tract lesion. This observation illustrates the interest of the use of LEPs in the diagnosis of neurological disease.
Since LEPs evaluate the small-diameter nerve fibres and the spinothalamic tracts, then similar results of LEPs and thermal sensory testing could be
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